Types of Elimination and Nucleophilic substitution
SN1 – Nucleophilic Substitution, Unimolecular
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This is a 2 step reaction.
1st step is rate determining (RDS)
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Only 1 species is present in RDS
= a unimolecular reaction.
SN2 – Nucleophilic Substitution, Bimolecular
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This is a 1 step reaction.
Both reagent and substrate present in RDS = a bimolecular reaction.
Step 1
Requirements
RDS
Requirements
RDS
Methyl, primary or
secondary alphacarbon
Secondary or tertiary
alpha-carbon
Good leaving group
Step 2
Stereochemistry at alpha-carbon is inverted
Nucleophilic reagent
1st step in SN1 is the same as the 1st step in E1. SN1 and E1 compete.
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Strong bases are usually (but not always) nucleophilic.
SN2 and E2 may compete.
E1 – Elimination, Unimolecular
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·
This is a 2 step reaction.
1st step is rate determining (RDS)
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Nucleophilic reagent
Watch out for OH leaving groups – they only come off when an acid is
present
Stereochemistry at
alpha-carbon is lost
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Good leaving group
E2 – Elimination, Bimolecular
Only 1 species is present in RDS
= a unimolecular reaction.
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·
This is a 1 step reaction.
Both reagent and substrate present in RDS = a bimolecular reaction.
Step 1
RDS
Requirements
Requirements
RDS
Secondary or tertiary
alpha-carbon
Step 2
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Mechanism Inspector is provided by The Royal
Society of Chemistry and supported by Shire.
Abstractable H atom
on beta-carbon
Good leaving group
Abstractable H atom
on beta-carbon
1st step in E1 is the same as the 1st step in SN1. SN1 and E1 compete.
A strong base
Watch out for OH leaving groups – they will deprotonate instead
·
·
Strong bases are usually (but not always) nucleophilic.
SN2 and E2 may compete.
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